Modulators may be used to control the generation of high voltage pulses for supply across a load, for example a magnetron. In the case of a magnetron load, such modulators will have components i.e. those directly connected to magnetron, at very high potentials, which must be isolated from components at lower potentials. For example, it is conventional to operate magnetrons with the main body forming the anode at earth potential and with the cathode and cathode heater to be at a high negative potential. This requires the cathode heater to be powered via a high voltage isolation barrier.
Such modulators and magnetrons may be used in linear accelerator systems, known as linacs, for x-ray generation. Such linacs can be used in medical applications, such as radiotherapy systems, and industrial application such as cargo scanning. In medical applications, very accurate control of the output of the magnetron is required as the output will affect the final dose received by the patient.
Some cathode heaters are operated at mains frequency via a mains transformer that has the required high voltage isolation. In such a system the operating frequency of the magnetron will vary as the AC heater current causes a varying magnetic field in the heater coil that will interact with the magnetic field of the magnetron magnet. Also, the low frequency of mains distribution can mean that AC heating can cause undesirable resonance within the magnetron structure leading to failure. On systems that require frequency stability throughout the magnetron pulse, for example in medical applications, it is conventional to rectify and smooth the isolated AC supply to the cathode heater. This is normally done at high frequency, normally greater than 100 kHz, to reduce the capacitance and therefore the size of the capacitors required to reduce the heater supply ripple to an acceptable level. Other cathode heaters use DC heating so as to avoid the issues associated with AC heating.